Heretofore, when placing a conduit hole in a wall of an electrical box for the installation of an electrical conduit, it was necessary for the electrician to determine the diameter of the electrical conduit and mark the exact center of the conduit on the box wall. This was usually done by measuring the diameter of the conduit and transferring it to the box side wall with a rule and straight edge. The hole is formed by a conventional knockout set which includes a circular die and cylindrical anvil interconnected by a threaded bolt. A hole is drilled through the box wall at the point marked as the center of the conduit. The die is placed on the outside of the box against the wall; the cylindrical anvil is placed on the opposite side of the box wall, and the bolt extends through the anvil, the drilled hole in the box wall and into the die. By tightening the bolt, the die is drawn through the box wall and into the cylindrical anvil to thereby form a conduit opening in the box wall, which should be positioned correctly for the installation of the electrical conduit if the center of the conduit was laid out correctly on the box wall. Through experience, this method of determining the exact center of a conduit has been found to be time consuming and inexact.